What is missing in many cases is a concepts guide, explaining the key ideas, how to combine things, and what's possible in various subject areas.
For GUI programs, menus / toolbars used to be the concept guide: what they show is what's possible, and they offer context help. This is why a GUI feels friendly. It sucks at composability, though. Current mobile interfaces, unfortunately, tend to lack this.
If tiny GUI-oriented programs were easy to compose, had an easy way to save the composed state, and a number of daily-use programs bundled with an OS came in this form, providing example and reference, many people would consider following the suit, I suppose.
This simple fact seems like the key to getting the masses into computing. For something like 6 years (say 12-18) GUIs were the way I interacted with computers. Need to do something and learn about it? Go and explore the UI until you find the option. If the option has a shortcut printed, you will remember it eventually.
Sadly, GUI design is a quite separate discipline from software design. This means much open source software is missing GUIs. Those who write the software aren't always GUI designers. This also creates the mismatch between composing software and composing GUIs. As they are different disciplines, what it means to combine them means different things.
A decent stopgap is massive frameworking and standardization on GUI to make it easier for devs to get a GUI.
To get the really good stuff, commercial entities have the best position. They need their stuff to be usable by everyone, and this finances the hiring of GUI people.
There is the rare gen of a developer that can also do GUI right, but that only has value in the case of small projects. When projects grow, unless all devs have the GUI knack, you're gonna need some dedicated GUI people.
It would be great if we could get more GUI-oriented people into open-source stuff but it seems like they aren't as attracted to open-source as devs are. It might be because devs can be at the ground floor of a project, and GUI, almost by neccesity, comes later.
"The Art of Unix Programming"
It's not perfect - I'd love to see a guide with more practical examples - but it does do a good job covering the basic philosophy and some of the history of why certain design decisions were made.